High temperature palming of ligated fibrous mats



United States Patent US. Cl. 28-74 Claims ABSTRACT OF THE DISCLOSURE A ligated fibrous mat of spontaneously elongatable synthetic fibers, such as polyethylene terephthalate, is heated to a temperature above about 200 C. During heating, the mat is restrained so as to prevent a surface area increase and to keep a constant density. After treatment,

the mat exhibits improved levelness from edge to edge and possesses smooth surfaces.

Background of the invention This invention concerns a process for improving the surface characteristics of a ligated fibrous mat and more particularly to a process for improving the surface smoothness and the edge to edge levelness of the mat.

In US. Patents 3,067,482 and 3,067,483 issued to I. L. Hollowell on Dec. 11, 1963 and US. Patent 3,100,721 issued to E. K. Holden on Aug. 13, 1963, methods for the preparation of moisture permeable sheet materials are described. Basically, these methods comprise forming a ligated fibrous mat and, thereafter, impregnating the mat with a microporous polymer impregnant. While the methods therein described illustrate the basic features of preparing useful moisture permeable sheet materials, improvements have since been made. An improvement has been the use of spontaneously elongatable fibers in preparing the ligated fibrous mat. By using such fibers and subsequently conventionally palming the mat (i.e., heating the mat between a heated drum and a tension controlled blanket to a temperature between about 90 C. and 140 C.), the pore distribution can be made more uniform and at the same time reduced to a smaller average size.

While conventional palming methods improve the interior mat structure, the surfaces of the mat remain comparatively rough and the mat is not level from edge to edge. Both of these features are highly undesirable. An unlevel mat with a rough surface will, after polymer impregnation, give an unlevel and rough surfaced impregnated sheet material which it is then necessary to smooth and level by bufiing. In general, this latter operation results in a loss of about of the impregnated mat.

Summary of the invention According to the present invention there is provided a process for improving the levelness and surface smoothness of a ligated fibrous mat comprised of spontaneously elongatable synthetic fibers. This process involves heating the mat at a temperature and for a time suificient to spontaneously elongate the fibers while at the same time restraining the mat such that there is no substantial increase in thickness or surface area of the mat. To accomplish the desired surface smoothness, the fiber elongation necessary in the process of this invention is at least that amount which would result in area growth of the mat were it not restrained. Preferably, the fiber elongation is equivalent to an unrestrained area growth of at least A mat treated according to the process of this invention possesses exceptionally smooth surfaces and is level from edge to edge. These features are illus- 3,503,107 Patented Mar. 31, 1970 trated by the fact that a mat, so treated, exhibits less than a 10% loss in yield on bufiing after polymer impregnation.

Description of the invention For the purposes of this invention, the percent of unrestrained area growth is based on the increase in surface area which the mat would undergo, due to high temperature spontaneous elongation, as compared with the surface area of the web prior to such elongation. The unrestrained area growth measurement is made on a dry web, i.e., a web having a moisture content of no more than one weight percent.

A variety of fibers capable of spantaneous elongation can be used in preparing the ligated fibrous mat. While spontaneously elongatable polyester fibers are preferred, the phenomena of spontaneous elongation has been observed in other synthetic organic polymers as well. For example, a polyamide prepared by reacting p-xylylene diamine and azelaic acid has been observed to be spontaneously elongatable, as have polyurethanes. Spontaneously elongatable addition type polymer fibers such as acrylics and polyolefins can also be used. In addition, the mat can be comprised of blends of various spontaneously elongatable fibers as well as blends of such fibers with non-spontaneously elongatable fibers.

Particularly preferred spontaneously elongatable fibers are those of polyethylene terephthalate prepared as described in the following two U.S. patents to R. E. Kitson and C. E. Reese: 2,931,068, issued Apr. 5, 1960; and 2,952,879 issued Sept. 20, 1960. A mat consisting essentially of fibers prepared according to these patents wherein the fiber itself has an unrestrained linear expansion of at least 20% at C., is especially preferred for use in this invention. A mat consisting of the aforementioned polyethylene terephthalate fibers exhibits an unrestrained area growth of at least the desired 25% at a temperature above 200 C. Of course, as previously mentioned, mats of other spontaneously elongatable fibers as well as various fiber blends can be used. The mat, however, must be capable, at an elevated temperature, of an unrestrained area growth of at least 25%.

The choice of a fiber denier, fiber length, or mat density, is not critical in this invention in order to obtain levelness and a smooth surface. However, the selection of these parameters is frequently important for the particular end use product. In general, when the mat is used in the preparation of the aforementioned moisture permeable sheet material, the denier, prior to the high temperature palming step of this invention, is generally about 1 to 3. Similarly, the fiber length at the same stage is between about 1 and about 2 inches though fibers having smal er or greater lengths can also be used. The mat density is generally between .20 and .30 gm./cc. while it is preferred to use a mat with a density between .23 and .26 gm./cc.

As with the above-described parameters, the particular method of preparing the ligated fibrous mat is not limited. Methods, such as carding and subsequent needle punching, as described in the US. Patent 3,067,482 are generally suitable.

The amount of spontaneous elongation that a fiber will undergo is temperature dependent and, thus, the total fiber e ongation is determined by the highest temperature the fiber is subjected t .oIn general, the fibers use ful in this invention show some spontaneous elongation at a temperature of about C. with maximum irreversible elongation being reached at a temperature of about 200 C. to 235 C., depending on the fiber. Since fiber elongation can be initiated at a comparatively low temperature, some spontaneous elongation of the fibers comprising the mat can occur prior to the high temperature palming step of this invention. In particular, some prior low temperature elongation will have occurred if the mat has been conventionally palmed at an earlier stage. In any event, to obtain the improved surface qualities of the ligated fibrous mat, the amount of spontaneous elongation in the present high temperature restrained palming step is at least as much as would result in a 25% area growth of the mat were it not restrained.

The palming operation of this invention can be accomplished in a variety of Ways. One method is to simply place the mat between a pair of heated plates which are spaced apart the approximate thickness of the mat and are appropriately shimmed on all sides to prevent lateral mat expansion. The temperature of the plates and dwell time is chosen so as to uniformly heat the mat to a temperature such that, were it not restrained between the plates and shims, it would exhibit a 25% area growth. For most mats, heating to a uniform temperature of about 150 C. to 235 C. is sufiicient to accomplish the necessary fiber elongation with temperatures above 200 C. being preferred.

Due to economic considerations continuous methods are preferred for palming the mat in the present invention. One method comprises transporting the mat on a porous carrier screen between two perforated parallel plates. The plates form an aperture for passage of the mat of about the thickness of the web and are shimmed on each side to prevent lateral expansion. Air heated to at least the temperature at which fiber elongation is to be accomplished is continuously forced through one of the perforated plates, generally the top, and thereafter through the mat. After spontaneous elongation has been accomplished, as determined by the air temperature and dwell time, which in turn is controlled by the porous carrier screen speed, the Web is cooled to ambient temperature. Care is taken in handling the hot mat due to its fragile nature.

In order to minimize mat distortion, Teflon tetrafluoroethylene coated glass cloth carriers can be used on either side of the mat for transporting it between the plates. At the elevated temperatures under which spontaneous elongation is accomplished, the mat becomes soft and compressible and is easily conveyed thruogh the aperture formed by the parallel plates. With this arrangement, no porous screen conveyor is generally needed and the mat is heated by conduction from the parallel plates in lieu of forced air.

An additional continuous method involves conveying the mat on a rotating drum with a curvilinear plate, spaced a fixed distance from the drum, over a portion of the drurns surface area. As with the parallel plates the height of the aperture formed by the drum and plate approximates the thickness of the mat. Shims are again used to prevent lateral expansion and heating is accomplished by either forced air or conduction.

As above described, the high temperature fiber elongation is accomplished while the mat is restrained between a pair of surfaces that are spaced apart the mats approximate thickness. This type of high temperature palming is termed fixed aperture palming; and is distinguishable from earlier palming techniques which employed much lower temperatures and arbitrary belt tensions. In the present invention, for a mat having an initial thickness of less than about 0.3 inch, the expansion generally does not exceed based on the mat thickness prior to the fixed aperture palming. In general, the expansion is restrained to less than this amount and preferably is just about zero.

While a necessary feature in the present high temperature palming operation is the prevention of any substantial increase in mat thickness, i.e., more than the aforementioned 10%, the thickness and surface area Registered trademark of E. 1. du Pont de Nemours and Company,

can be decreased by the application of pressure during the high temperature palming operation. Such pressure palming provides a smooth mat surface, but is not generally used where it is desirable to maintain a constant mat density and a fibrous surface.

The following example illustrates the invention. All parts and percentages are by weight unless otherwise indicated.

Example 1 1.8 denier polyethylene terephthalate fibers having an approximate length of 1.5 inches are formed into a mat by blowing the fibers against a moving porous screen. The mat having a thickness of about 3 inches is then fed into a needle-loom and the ligated mat issuing therefrom with a thickness of about 0.5 inch is shrunk about 65% of its area by immersion in water at 65 C. Subsequently, the shrunk, ligated mat is dried at a temperature of 60 C. At this stage, the fibers in the mat are capable of spontaneous elongation and the mat is conventionally palmed between a heated drum and restraining blanket at a temperature of about C. On completion, the mat has an average thickness of about 0.195 inch (range 0185-0205) and a density of about 0.23 gm./ cc.

An 18 wide, 18 yard long section of this ligated, shrunk, and conventionally palmed mat is now subjected to high temperature fixed aperture palming. This is accomplished by placing the aforementioned Teflon-coated glass cloth carriers on either side of the mat and passing the mat between a pair of parallel plates spaced 0.195 inch apart and heated to a temperature of 205 C. The residence time of the mat between the plates is about two minutes. Shims placed between the plates on either side prevent lateral mat expansion. An equivalent unrestrained heating of the mat results in a mat area growth of about 35%.

Subsequently, the high temperature palmed mat is cooled to room temperature. The cooled palmed mat has an average thickness of 0.188 inch (range 0.185-0.l9'l) and a density of 0.23 gm./cc. The mat is then impregnated, coagulated, leached, and dried in the manner substantially as described in Example 1 of US. Patent 3,067,- 482 to yield an impregnated mat with a thickness of about 0.184 inch (range 0.180 0.190). The surfaces of this mat are then buffed with grit buffing paper until they exhibit no evidence of polymer skin. Bufiing results in a removal of 0.014 inch of impregnated mat and, consequently, a reduction in mat thickness to 0.17 inch. The yield of this impregnated mat, based on the impregnated unbutfed mat, is 92.4%

For comparison, an 18" wide, 18 yard long section is cut from the same ligated, shrunk, conventionally palmed mat described above. The high temperature palming step for this section is omitted and the ligated mat is directly impregnated, coagulated, leached, and dried as in Example 1. The dried impregnated mat has an average thickness of 0.193 inch (range 018241200) which decreases to an average thickness of 0.155 inch on buffing, indicating that 0.038 inch of impregnated mat is removed. The yield of this mat is 80.4%

We claim:

1. A process for improving the surface smoothness and levelness of a ligated fibrous mat comprised of spontaneously elongatable synthetic fibers comprising heating the mat while restraining it such that there is no substantial increase in mat thickness or surface area, the heating being at a temperature and for a time such that, were the mat not restrained, its surface area would increase at least 25 2. The process of claim 1 wherein the mat thickness and surface area remain substantially constant during heating.

3. The process of claim 2 wherein the mat is restrained during heating by passing it through a fixed aperture and heating is at a temperature and for a time such that, were the mat not restrained, its surface area would increase at least 35%.

4. The process of claim 3 wherein the mat consists of spontaneously elongatable polyethylene terephthalate fibers and heating is accomplished at at least 200 C.

5. The process of claim 4 wherein heating is accomplished at 200 C.-235 C.

6-. In the process for preparing a moisture permeable sheet material comprising the sequential steps of forming a ligated fibrous mat comprised of spontaneously elongatable synthetic fibers, impregnating the mat with a microporous polymer impregnant, and thereafter coagulating, leaching, and drying the mat, the improvement comprising, prior to the step of impregnating the mat, heating the mat while restraining it such that there is no substantial increase in mat thickness or surface area, the heating being at a temperature and for a time such that, were the mat not restrained, its surface area would increase at least 25%.

7. The process of claim 6 wherein the mat thickness and surface area remain substantially constant during heating.

8. The process of claim 7 wherein the mat is restrained References Cited UNITED STATES PATENTS 2,893,105 7/ 1959 Lauterbach 2872.2 3,156,965 11/1964 Howard 2872 3,407,461 10/ 1968 Stevenson et a1 2872.2

FOREIGN PATENTS 547,637 9/ 1942 Great Britain.

LOUIS K. RIMRODT, Primary Examiner US. Cl. X.R. 264324 

